The GW1500-DH485-ModTCP acts as is a Modbus Slave device to ModTCP Master on Ethernet which initiates communication with one or many Allen Bradley (AB) DH485 PLC Type devices.

The GW1500 Modbus slave accepts all Modbus TCP/IP master requests to remote Modbus Slave addresses 1 to 63 decimal, which correspond to PLC’s at DH485 nodes 1 to 77 octal. The PLC5/SLC500 type device which processes requests from the GW1000 must not be “protected” and the full range of elements in a particular file must have been previously created.

The Modbus read and write bit/coil commands are used with the Allen Bradley Binary (B) file type and word/register commands are used with the Allen Bradley Integer (N) file type. The GW1500 accepts the following MODBUS Commands: Coils – 01 and 05; Registers – 03,04,06,16; Inputs – 02

Modbus TCP Slave Lookup Translation Table Usage

The received Modbus Master request is examined against the configured slave lookup table. The search is done starting with the Data Type in the slave request, based on the Modbus command, and then the register range is checked based on the start register and size.

If the slot is a match, a DH485 Read/Write request is sent to the DH485 node corresponding to the Modbus slave address in the Master’s Request. The PLC File Number and Starting Offset in the table slot is then used to complete the DH485 Read/Write request to the PLC Type device.

Coil versus Register Support

For Coil support you must select the Allen Bradley PLC type you want to communicate with for that slot. PLC5 and SLC5/04′s support different DH485 commands so this has to be known. The number of Coils must be represented in 16 bit word groupings in the Lookup Table slot as this is the resolution of Allen Bradley B(binary) and N(Integer) file types.

Holding Register commands are supported by both the PLC5 / SLC5 so the “PLC or SLC” Option can be selected.

DH485 Function Support

The following DH485 functions will be issued to the PLC Type device:
For PLC5 or SLC support, a PLC5 Type Write and Read function (0×67 or 0×68) will be issued for Coil and Register Modbus commands. For PLC5 Coils, a Read/Modify word function (0×26) will be issued. For SLC Coil support, the Bit Write (0xAB) Function will be used.